Microporous membranes made of polymeric materials are used in various applications, for example, filter membranes and separation membranes for medical and industrial use, and separators, such as battery separators and condenser separators.
In particular, with the recent growing demand for secondary batteries as power supplies for mobile phones, mobile personal computers, and automobiles, there is also a growing demand for battery separators. However, battery separators made of conventional polymeric materials have insufficient characteristics, particularly unsatisfactory heat resistance.
Use of a blend of polyethylene (PE) and polypropylene (PP) or a higher-molecular-weight polyethylene (PE) to improve the heat resistance of battery separators has resulted in still insufficient characteristics and particularly cannot achieve high heat resistance.
Patent Literature 1 describes a high-molecular-weight PP having an MFR≦1.2 g/10 min. However, the effect is unclear because Example fails to provide any data verifying the description.
Patent Literature 2 describes a separator made of polyolefins having different melting points. However, as shown in an example, a separator made of a PP having an MFR of 3 g/10 min and a high-density polyethylene (HDPE) having an MFR of 5.5 g/10 min does not have desired properties as a separator (for example, the upper limit temperature at which pores remain closed).
Patent Literature 3 describes a microporous membrane (separator) made of polyolefins having different viscosity-average molecular weights. However, a high-molecular-weight PE only is used, and there is no description of the properties of the separator.
Patent Literature 4 describes a microporous membrane (separator) made of a polyolefin, particularly a metallocene PE, having a residual Cl amount of 5 ppm or less and a viscosity-average molecular weight of 1,000,000 or more. However, the examples describe only a metallocene PE, and desired properties (for example, puncture strength at 150° C.) can not be achieved.
Patent Literature 5 describes a microporous membrane (separator) made of a PE having a viscosity-average molecular weight (Mv) of 300,000<Mv<600,000, a PE having a Mv of 600,000≦Mv≦10,000,000, and a PP (150,000≦Mv≦700,000). However, the microporous membrane (separator) does not have desired properties as a separator (for example, thermal fracture temperature).
Patent Literature 6 describes a microporous membrane (separator) made of a PE and a PP having a weight-average molecular weight (Mw) of 500,000 or more. However, the weight-average molecular weight Mw of PPs in an example is approximately 860,000 (MFR=0.4 g/10 min) at the maximum, and desired properties (for example, meltdown temperature) cannot be achieved.
Patent Literature 7 describes a microporous membrane (separator) made of two polyolefins each having a weight-average molecular weight (Mw) of 500,000 or more. However, the examples describe only HDPE, and desired heat resistance cannot be achieved.
As described above, application of high-molecular-weight polypropylenes to improve, for example, the heat resistance of microporous membranes has been conducted but had insufficient effects.